1. Field of the Invention
The present invention generally relates to integrated multi-layered microwave circuits used in a mobile radio communication system utilizing a radio wave of a microwave band such as a vehicle information and communication systems (VICS) and, more particularly, is directed to an integrated multi-layered microwave circuit and a method of fabricating it which is easily miniaturized and has less signal loss.
2. Description of the Related Art
In the VICS, a vehicle antenna receives various information such as a location of a vehicle or a road condition such as congestion which is transmitted from a terrestrial station such as a beacon provided on a road, and various devices mounted on the vehicle are controlled based on the received information. Further, it has been proposed to arrange the VICS so that the vehicle antenna transmits information from the various devices mounted on the vehicle to an external station such as the earth station or another vehicle. The thus constituted VICS has been put to practical use as an example of the mobile radio communication systems utilizing a radio wave of a microwave band. In particular, it has been an important problem to be solved to miniaturize and decrease weight in a radio communication terminal for a mobile station which is mounted in a movable body (mobile) or vehicle and processes a signal received by a vehicle antenna.
In a conventional circuit arrangement usable as the radio communication terminal for the mobile station of the mobile radio communication system, an antenna portion and a communication portion for processing a signal received by the antenna portion are provided separately, in general, in order to freely position the antenna portion. That is, in almost cases, the antenna portion and the communication portion are located at different portions. For example, in the prior art disclosed in JP-A-2-152304, an antenna for a mobile radio communication and a communication portion are located at different portions and are connected to each other through a coaxial cable.
Further, there has been proposed various circuits in each of which an antenna for a mobile communication for receiving a radio wave of a microwave band is constituted by a micro strip line and integrated with a communication portion for processing the received signal. One is a circuit in which an antenna portion and an integrated circuit constituting a communication portion are separately fabricated and then integrated as described in JP-A-63-316905. Another is a circuit in which an antenna constituted by a micro strip line and a communication portion constituted by a semiconductor circuit are formed on the same major surface of a substrate as described in JP-A-1-112827.
In the circuit described in JP-A-2-152304, since the coaxial cable is used to connect the antenna for the mobile communication and the communication portion, a radio frequency (RF) signal is attenuated due to a signal loss in the coaxial cable, thereby an efficiency of the communication portion degrades.
Further, the circuit described in JP-A-63-316905 has the disadvantage such that, since material of the communication portion, i.e., semiconductor material is different from material of the antenna portion, i.e., conductive material, it is required, in an integrating process of the materials, to provide a conductive layer at a rear surface of the communication portion and to utilize an adhesive solder, thereby complicating an assembling process of the circuit.
The circuit described in JP-A-1-112827 is advantageous in that signal loss of an RF signal can be decreased and the circuit can be miniaturized and light-weighted since the antenna and the communication portion are integrally formed. However, this circuit has the disadvantage such that since both the antenna and the communication portion are located on the same surface, a radio wave having the same frequency of that from the antenna is transmitted from the communication portion and interferes with a radio wave transmitted from the antenna, so that this circuit can not be applied to one whose communication portion has a modulating function.
Accordingly, it is an object of the present invention to provide an improved integrated multi-layered microwave circuit in which the aforementioned shortcomings and disadvantages encountered with the prior art can be eliminated.
More specifically, it is an object of the present invention to provide an integrated multi-layered microwave circuit which can integrate an antenna portion and a communication portion by using a conventional fabricating process of a circuit substrate and which is small in size and weight, less in signal loss and high in efficiency.